Method for producing methylmethacrylate

ABSTRACT

The present invention relates to a process for producing methyl methacrylate, comprising the following steps: A) producing methacrolein and B) reacting the methacrolein obtained in step A) in an oxidative esterification reaction to give methyl methacrylate, characterized in that the two steps A) and B) take place in a liquid phase at a pressure of from 2 to 100 bar, and step B) is carried out in the presence of a heterogeneous noble-metal-containing catalyst comprising metals and/or comprising metal oxides.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of Ser. No. 15/795,887,filed Oct. 27, 2017, which is a continuation application of Ser. No.14/784,320, now granted as U.S. Pat. No. 9,890,105, filed Oct. 14, 2015which is a National stage entry of PCT/EP2014/057380 filed Apr. 11,2014, which claims priority to European Patent Application No.13002076.1 filed Apr. 19, 2013. The contents of these applications areincorporated herein by reference in their entirety.

The present invention relates to a process for producing methylmethacrylate by direct oxidative esterification of methacrolein and theproduction of methacrolein.

Large quantities of methyl methacrylate are used for producing polymersand copolymers with other polymerizable compounds. Methyl methacrylateis moreover an important building block for various speciality esters ofmethacrylic acid (MAA), these being produced by transesterification withthe corresponding alcohol.

It is therefore highly desirable that the said starting material can beproduced by a process which is as simple as possible, and cost-effectiveand protects the environment.

Methyl methacrylate (MMA) is nowadays mostly produced from hydrogencyanide and acetone by way of the resultant acetone cyanohydrin (ACH) asmain intermediate. This process has the disadvantage of producing verylarge amounts of ammonium sulphate, treatment of which incurs very highcosts. Other processes not based on ACH are described in the relevantpatent literature and are also carried out on a production scale. Amongthe raw materials used in this context as starting materials are thosebased on C-4 compounds, for example isobutylene or tert-butanol, whichare converted by way of a plurality of stages to the desired methacrylicacid derivatives.

The general procedure here is that isobutylene or tert-butanol isoxidized in a first stage to give methacrolein, which is then reactedwith oxygen to give methacrylic acid. Methanol is then used to convertthe resultant methacrylic acid to MMA. Further details of the saidprocess are described inter alia in Ullmann's Encyclopedia of IndustrialChemistry 2012, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, MethacrylicAcid and Derivatives, DOI: 10.1002/14356007.a16_441.pub2, and in Trendsand Future of Monomer-MMA Technologies, SUMITOMO KAGAKU 2004-II.

Ethylene can also be used as starting material instead of a C4 buildingblock such as isobutylene in one variant of said production method, andis first reacted with synthesis gas to give propanal; reaction withformaldehyde then gives methacrolein. The resultant methacrolein isoxidized by air in the gas phase on a heterogeneous catalyst to givemethacrylic acid, which is esterified with methanol to give MMA(Ullmann's Encyclopedia of Industrial Chemistry 2012, Methacrylic Acidfrom Ethylene, and Trends and Future of Monomer-MMA Technologies,SUMITOMO KAGAKU 2004-II). This process has been operated since 1990 byBASF in a plant with a capacity of 40000 metric tons per annum formethacrylic acid production. According to the SUMITOMO article, thisprocess was developed by BASF for specific requirements, and it istherefore difficult to make general use of said process for producinglarger quantities of MMA.

Another process obtains MMA through oxidation of isobutylene ortert-butanol with atmospheric oxygen in the gas phase on a heterogeneouscatalyst to give methacrolein followed by use of methanol in anoxidative esterification reaction of methacrolein. This process,developed by ASAHI, is described inter alia in the publications U.S.Pat. Nos. 5,969,178 and 7,012,039. This process is also described in theSUMITOMO article, which provides detailed information about thedisadvantages of said process, consisting in particular in high energyusage, arising inter alia because of an unpressurized procedure.

In addition, other problems associated with all the processes describedabove are in particular the relatively unsatisfactory yield, high lossesin the oxidation steps and attendant CO₂ formation, and in general termsthe attendant formation of by-products requiring complicated steps toisolate the product: all of the processes starting from isobutylene orfrom equivalent C-4-based raw materials, such as TBA or MTBE, usinggas-phase oxidation on a heterogeneous catalyst system achieve yieldsbelow 90%, and the relevant literature describes yields below 85% formethacrolein production starting from isobutylene (e.g. Table 5 inUllmann's Encyclopedia/Sumitomo, see above). The gas-phase processnaturally proceeds at moderate pressures of from 1 to 2 bar absolute,and produces a process gas which comprises only about 4-6% by volume ofthe product component. Isolation of the useful product from the inertgas ballast incurs accordingly high energy cost and consumes largeamounts of cooling energy, as well as steam for multi-stage distillativework-up steps.

Production of MMA according to the methods described hitherto producesrelatively large amounts of wastes, in particular exhaust gases orwastewater, which require expensive disposal.

Conduct of some of the processes described above moreover requires verycomplex, and therefore expensive, plant, with associated high capitalexpenditure and high maintenance costs.

The overview article cited above from SUMITOMO describes the respectivedisadvantages in detail, and can therefore be incorporated herein by wayof reference.

Furthermore, Patent Application CN 101074192 describes an MMA productionprocess in which methacrolein is first prepared from propanal andformaldehyde at a temperature in the range from 40 to 45° C. and with areaction time in the range from 30 to 100 minutes, and is then oxidizedwith methanol to give MMA. In addition, a similar process is proposed byYuchao Li et al. “Synthesis of methacrolein by condensation ofpropionaldehyde with formaldehyde”, Advance Materials Research Vols.396-398 (2012). pp. 1094-1097. Said publication expressly advisesavoidance of operation at elevated temperature or at superatmosphericpressure. Said process has the disadvantage of a high requirement onacid and amine, these being used to catalyse the reaction. Consequentlylarge amounts of waste products are produced, since a substantialproportion of the amine is destroyed under the conditions mentioned. Oneof the side reactions, which deactivates the catalyst, is theEschweiler-Clarke reaction which leads to formation of methylatedtertiary amine which is then not capable of catalysing the Mannichreaction (U.S. Pat. No. 4,408,079, column 2, lines 15 ff): by way ofexample, dimethylamine becomes trimethylamine.

If, as described by Li, operations are then carried out at or in thevicinity of atmospheric pressure with large stoichiometric amounts ofcatalyst base, increased deactivation of the catalyst occurs, and theresultant procedure is therefore not cost-effective. These problemsincur high costs which make the process described relatively uneconomic.The long reaction time resulting from unpressurized conduct of thereaction is another serious disadvantage of both of these processes.

EP 0 890 569 discloses a process for the production of methylmethacrylate by direct oxidative esterification of methacrolein withmethanol. EP 0 890 569 explicitly teaches here that a low water contentof less than 2% by weight, preferably less than 1% by weight, in themethacrolein is essential for the oxidative esterification. The exampleslist exclusively reactions with water content below 0.8% by weight.Furthermore, according to this teaching it is important that the totalcontent of contaminants must be small. EP 0 890 069 therefore teachesoxidative production of methacrolein in the gas phase from isobutylenewith oxygen, and then complicated dewatering of the methacrolein in acolumn.

Although EP 0 092 097 and DE 28 55 504 teach an alternative synthesisprocess for methacrolein in the liquid phase, in a process wherepropanal is reacted with formaldehyde, this process produces a largeamount of water which according to the teaching of EP 0 890 569 rendersthis type of process unsuitable without complicated purification asprecursor stage for the oxidative esterification of methacrolein tomethyl methacrylate. The starting materials and by-products used in thisprocess, for example dimeric methacrolein, also potentially exert ayield-reducing or detrimental effect in oxidative esterification,alongside the high water content: the product according to DE 28 55 504comprises more than 5% by weight of an aldol by-product which would bedetrimental in oxidative esterification and would inevitably requirecomplicated purification of the methacrolein.

In the light of the prior art, an object of the present invention istherefore to provide a technically improved MMA production process whichdoes not have the disadvantages of conventional processes.

A particular objective is to enable the production of MMA with arelatively low energy usage. Furthermore the process shall be carriedout in a manner that provides a high level of protection of theenvironment, so that the quantities of wastes obtained are very small. Aparticular object of the present invention is to improve the overallyield of MMA, based on the raw materials used, for example bydiscovering and combining individual reaction steps with high productselectivity.

Moreover, it should be possible to carry out the process with a verysmall number of steps, which should be simple and reproducible.

In addition, it should be possible to carry out the process by usingrelatively simple and inexpensive plant. Capital expenditure for theplant should accordingly be small. Maintenance of this plant should besimple and inexpensive.

Other objects not explicitly mentioned are apparent from the overallcontext of the description and claims hereinafter.

A process with all of the features of patent claim 1 achieves the aboveobjects, and also achieves other objects which are not explicitlymentioned, but which are readily derivable or deducible from thecircumstances discussed in the introduction of this specification.Dependent claims 2 to 18 protect advantageous embodiments of the claimedMMA production process.

Accordingly, the present invention provides a process for producing MMA,comprising the following steps:

A) producing methacrolein from propanal and formaldehyde and

B) reacting the methacrolein obtained in step A) in an oxidativeesterification reaction to give MMA, which is characterized in that thetwo steps A) and B) take place in a liquid phase at a pressure of from 2to 100 bar, and step B) is carried out in the presence of aheterogeneous noble-metal-containing catalyst comprising metals and/orcomprising metal oxides.

By virtue of the claimed process it is possible, in a manner that wasnot foreseeable, to provide an MMA production process which does nothave the disadvantages of conventional processes. Surprisingly, it hasbeen found here that, contrary to the general teaching of the prior art,steps A) and B) can also be combined without any complicated interveningpurification or dewatering of the methacrolein, and lead to high methylmethacrylate yields.

In particular, MMA can be produced with relatively low energy usage. Theprocess can moreover be carried out in a manner that provides a highlevel of protection of the environment, giving relatively smallquantities of wastes and substantially increasing atom efficiency.

In particular, neither of the reaction steps A) and B) in the claimedprocess requires that additional water be introduced into the reactionmixture and then in turn removed, and overall reaction volumes andvolume flow rates can therefore be kept low.

The process can moreover be carried out with relatively few steps, andthese are simple and reproducible and proceed with relatively highspace-time yields.

Catalyst requirement, in particular organic base requirement per metricton of MMA produced, is moreover very small.

The process can moreover be carried out with relatively simple andinexpensive plant. Capital expenditure for the plant is low. Maintenanceof this plant is simple and inexpensive.

In this context it should be stated that, in comparison with theprocesses according to the prior art, the number of return streams andthe magnitude of these have been reduced.

The by-products produced can moreover be removed very simply from thereaction mixtures, and the overall process can therefore be carried outwith high yield, without any need for complicated purification steps.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an example of a diagram of the claimed process.

FIG. 2 shows a possible system for the reaction of formaldehyde withpropanal to give methacrolein.

FIG. 3 shows a possible apparatus which can produce MMA from MAL.

Step A)

The claimed process comprises the production of methacrolein. Theprocesses suitable for this purpose are known to the person skilled inthe art and are subject matter of relevant overview articles, forexample in Ullmann's Encyclopedia of Industrial Chemistry 2012,Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, Acrolein and Methacrolein,DOI: 10.1002/14356007.a01_149.pub2.

The first stage of the process of the invention comprises the reactionof propanal with formalin to give methacrolein.

Particularly suitable processes are those in which the total amount ofseparately added water or, respectively, water vapour during the conductof the process is not greater than 100 mol %, preferably 50 mol %,particularly preferably 30 mol %, and most preferably 10 mol %, based ineach case on methacrolein. The most suitable processes for producingmethacrolein are those where there is no reaction step in which water isadded separately to the reaction mixture. The water introduced with thereactants and/or catalysts, as well as the produced water of reactionare here excluded.

Surprisingly, however, the water content is not very relevant, or atconcentrations of up to 5% by weight has almost no adverse effect on theoxidative esterification of step B). In the light of the cited prior artit is therefore in particular surprising that methacrolein synthesisfrom propanal and formaldehyde can be combined with oxidativeesterification, and indeed that it is optionally possible to omit anydewatering after step A).

The reaction, which is achieved by way of an aldol condensation orMannich condensation, is not per se critical. However, preferredprocesses are those which feature high yield and low by-productformation.

It is therefore preferable to use reactions which have a selectivity ofat least 80%, preferably at least 90% and particularly preferably atleast 92%, based on the amount of propanal used.

Moreover, preference is given to reactions which have a high yield andhigh conversions by a single passage through the reaction zone.Preferred reactions feature a yield and conversions of at least 80%,preferably at least 90% and particularly preferably at least 92%, basedon the amount of propanal used.

It can also be provided that the reaction according to step A) takesplace with a molar ratio of propanal to formaldehyde which is preferablyin the range from 2:1 to 1:2, particularly preferably from 1.5:1 to1:1.5 and specifically preferably from 1.1:1 to 1:1.1. It is veryparticularly preferable to use an equimolar ratio of propanal toformaldehyde. It is thus possible, in particular at high conversions, toomit any removal and return of propanal and/or formaldehyde from themixture obtained after the reaction according to step A).

The reaction of propanal with formaldehyde generally uses catalysts, andvarious systems are known here which lead to a high yield ofmethacrolein, with high selectivity.

Preferred processes for producing methacrolein starting from propanaland formaldehyde are described inter alia in the publications U.S. Pat.No. 7,141,702; DE 32 13 681 A1; U.S. Pat. Nos. 4,408,079; 2,848,499; JP4173757A (JP 19900300135); JP 306942062 and EP 0 317 909 A2, and forpurposes related to disclosure the teaching of the said publications ishereby incorporated by way of reference into the present application.

The reaction of propanal with formaldehyde is carried out in thepresence of acid, generally inorganic acid or organic mono-, di- orpolycarboxylic acid, preferably monocarboxylic acid, in particularaliphatic monocarboxylic acid.

Carboxylic acids advantageously used are aliphatic monocarboxylic acidshaving from 1 to 10, preferably from 2 to 4, carbon atoms, or di- andpolycarboxylic acids having from 2 to 10, preferably 2 and from 4 to 6,carbon atoms. The dicarboxylic acids and polycarboxylic acids can bearomatic, araliphatic, and preferably aliphatic, carboxylic acids.Suitable examples are acetic acid, propionic acid, methoxyacetic acid,n-butyric acid, isobutyric acid, oxalic acid, succinic acid, tartaricacid, glutaric acid, adipic acid, maleic acid, and fumaric acid. Inprinciple, it is equally possible to use other organic acids, but theyare generally less advantageous for reasons of price. Inorganic acidsused are generally sulphuric acid and phosphoric acid. Acid mixtures canalso be used.

It is particularly preferable to use at least one organic acid for thereaction of propanal and formaldehyde, and even more preferably aceticacid.

The proportion of acid, based on propanal, is from 0.1 to 20 mol %,advantageously from 0.5 to 10 mol %, preferably from 1 to 5 mol %.

The reaction of propanal with formaldehyde is carried out in thepresence of organic bases, preferably amines, particularly preferablysecondary amines. Amines that can be used are preferably those of theformula R¹R²NH, where R¹ and R² are identical or different and arerespectively an alkyl moiety having from 1 to 10, advantageously from 1to 8, in particular from 1 to 4, carbon atoms, which can also havesubstitution by ether, hydroxy, or secondary or tertiary amino groups,in particular by from 1 to 2 of said groups, or are an aralkyl moietyhaving from 7 to 12 carbon atoms or a cycloalkyl moiety having from 5 to7 carbon atoms, and R¹ and R² can also, with the adjacent nitrogen, bemembers of a heterocyclic, advantageously 5- to 7-membered ring whichcan also comprise another nitrogen atom and/or an oxygen atom and whichcan have substitution by hydroxyalkyl or alkyl groups having from 1 to 4carbon atoms.

Examples of amines that can be used are: dimethylamine, diethylamine,methylethylamine, methylpropylamine, dipropylamine, dibutylamine,diisopropylamine, diisobutylamine, methylisopropylamine,methylisobutylamine, methyl-sec-butylamine, methyl(2-methylpentyl)amine,methyl(2-ethylhexyl)amine, pyrrolidine, piperidine, morpholine,N-methylpiperazine, N-hydroxyethylpiperazine, piperazine,hexamethyleneimine, diethanolamine, methylethanolamine,methylcyclohexylamine, methylcyclopentylamine, dicyclohexylamine orappropriate mixtures.

It may also be provided that at least one of the amines used has nohydroxy group. It is particularly preferable that the proportion ofamines having at least one hydroxy group is at most 50% by weight,preferably at most 30% by weight, and particularly preferably at most10% by weight, based on the weight of the amines used.

The proportion of organic base, preferably of secondary amines, is from0.1 to 20 mol %, advantageously from 0.5 to 10 mol %, preferably from 1to 5 mol %, based on propanal.

The ratio of the equivalents of amine to acid is preferably selected insuch a way as to give a resultant pH of from 2.5 to 9 in the reactionmixture prior to the reaction.

It can also be provided that the molar ratio of acid to organic base,preferably amine, is in the range from 20:1 to 1:20, preferably in therange from 10:1 to 1:10, particularly preferably in the range from 5:1to 1:5 and specifically preferably in the range from 2:1 to 1:2.

The reaction temperature for the reaction of propanal with formaldehydeat the exit from the reaction zone is from 100 to 300° C., preferablyfrom 130 to 250° C., with preference from 140 to 220° C., in particularfrom 150 to 210° C.

The reaction pressure is in the range from 2 to 300 bar, preferably from5 to 250 bar, particularly preferably from 10 to 200 bar, advantageouslyfrom 15 to 150 bar, preferably from 20 to 100 bar and in particular from40 to 80 bar. Pressure and temperature are adjusted in such a way thatthe reaction always takes place below the boiling point of the reactionmixture, i.e. the reaction proceeds in the liquid phase.

For the purposes of the present application, all pressure data areabsolute pressure in the unit bar.

The residence time or reaction time is preferably at most 25 minutes,advantageously from 0.01 to 25 minutes, more advantageously from 0.015to 10 minutes, preferably from 0.03 to 2 minutes. The residence time orreaction time is particularly preferably in the range from 0.1 to 300seconds, specifically preferably in the range from 1 to 30 seconds. Itis advantageous to use a tubular reactor as reactor at residence timesbelow 10 minutes. The residence time here refers to the time for whichthe reaction mixture is reacted. All of the components are present hereat reaction pressure and temperature, and the said time can therefore becalculated from the distance between the mixing point and thedepressurization point. The depressurization point is the point at whichthe mixture is brought from reaction pressure to a pressure below 5 bar.

The reaction mixtures can also comprise, alongside water, organicsolvents, e.g. propanol, dioxane, tetrahydrofuran, and methoxyethanol.

It can also be provided that the reaction of propanal with formaldehydeto give methacrolein according to step A) takes place in the presence ofpreferably at least 0.1% by weight, with preference at least 0.2% byweight and particularly preferably at least 0.5% by weight, of methanol,based on formalin. Despite the said relatively high methanolconcentrations, by virtue of the claimed conduct of the reaction for thesubsequent step B) it is possible to omit any complicated removal ofmethanol at the stage of formalin production and/or of methacroleinpurification.

According to one particular embodiment, formaldehyde and propanal can bemixed before the said starting materials are brought to reactionpressure and/or temperature.

The reaction can be carried out as follows: a mixture of propanal,amine, formaldehyde and advantageously water and/or acid and/or base iskept at the reaction temperature and the reaction pressure during thereaction time.

In one preferred embodiment, a mixture (advantageously equimolarmixture) of formaldehyde and propanal can be heated by way of a heatexchanger to the desired reaction temperature and passed into a tubularreactor. A catalyst solution (solution of the secondary amine and of anacid, advantageously in H₂O) optionally heated by way of a heatexchanger likewise to the reaction temperature can be injected at thereactor inlet into the said mixture. The strongly exothermic reactionbegins, and the temperature of the reaction mixture increases further.It is preferable that a pressure-retention valve at the reactor outletis used to keep the pressure under which the reaction proceeds at valuessuch that the reaction mixture still remains liquid during the reactiontime, even when temperatures in the reactor are high. After thereaction, the reaction mixture can be depressurized to atmosphericpressure and worked up. In the production of methacrolein from propanaland formaldehyde it is preferable that the reaction mixture is passedinto a column for steam-stripping. The methacrolein is dischargedtogether with water at the top of the column. The mixture is condensedand separated by way of a phase separator to give an upper and a lowerphase. The upper phase comprises the methacrolein, and is reacted byoxidative esterification according to step B) to give MMA. The lowerphase is composed mainly of water. It can preferably in turn be at leastto some extent returned to the column in order to remove the residualmethacrolein dissolved therein.

The aqueous catalyst solution can be drawn off at the bottom of thecolumn together with the water formed in the reaction and the water ofthe formaldehyde solution. For the purposes of further processing, theliquid from the bottom of the column can be discarded if very littleamine and/or acid is used and return of the catalyst is therefore thennot worthwhile.

However, in the case of higher concentrations of amine and/or of acid inthe material discharged at the bottom of the column it is also possibleto carry out distillative removal of water at least to some extent andin turn to return the catalyst solution to the reactor. Anotherpossibility is to divide the material discharged at the bottom of thecolumn into two sub-streams in such a way that one sub-stream comprisesprecisely the amount of water that was formed during the reaction andintroduced with the starting materials. Said sub-stream is then removedfrom the system, and the remaining proportion is returned to thereactor. Aqueous formaldehyde and propanal can also be separatelypreheated and introduced into the reactor.

The propanal used for producing methacrolein can be purchased in largequantities. The said compound can preferably be obtained by reaction ofethylene with carbon monoxide (CO) and hydrogen (H₂). Thehydroformylation reaction generally carried out for this purpose is wellknown, and reference is made in this connection to standard literature,for example Kirk-Othmer Encyclopedia of Chemical Technology, John Wiley& Sons, Inc., OXO Process and Franke et al., Applied Hydroformylation,dx.doi.org/10.1021/cr3001803, Chem. Rev. 2012, 112, 5675-5732; the saidpublications are hereby incorporated by way of reference into thepresent application.

Catalysts are usually used for this reaction. Among the preferredcatalysts are in particular compounds in which rhodium, iridium,palladium and/or cobalt is present, particular preference being givenhere to rhodium.

According to one particular embodiment, it is in particular possible touse, for the catalysis, complexes which comprise at least onephosphorus-containing compound as ligand. Preferredphosphorus-containing compounds comprise aromatic groups and at leastone, particularly preferably two, phosphorus atoms. Among thephosphorus-containing compounds are in particular phosphines,phosphites, phosphinites, phosphonites. Examples of phosphines aretriphenylphosphine, tris(p-tolyl)phosphine, tris(m-tolyl)phosphine,tris(o-tolyl)phosphine, tris(p-methoxyphenyl)phosphine,tris(p-dimethylaminophenyl)phosphine, tricyclohexylphosphine,tricyclopentylphosphine, triethylphosphine, tri(1-naphthyl)phosphine,tribenzylphosphine, tri-n-butylphosphine, tri-t-butylphosphine. Examplesof phosphites are trimethyl phosphite, triethyl phosphite, tri-n-propylphosphite, tri-i-propyl phosphite, tri-n-butyl phosphite, triisobutylphosphite, tri-tert-butyl phosphite, tris(2-ethylhexyl) phosphite,triphenyl phosphite, tris(2,4-di-tert-butylphenyl) phosphite,tris(2-tert-butyl-4-methoxyphenyl) phosphite,tris(2-tert-butyl-4-methylphenyl) phosphite, tris(p-cresyl) phosphite.Examples of phosphonites are methyldiethoxyphosphine,phenyldimethoxyphosphine, phenyldiphenoxyphosphine,2-phenoxy-2H-dibenzo[c,e][1,2]oxaphosphorine and its derivatives inwhich some or all of the hydrogen atoms have been replaced by alkyland/or aryl moieties or halogen atoms. Commonly used phosphinite ligandsare diphenyl(phenoxy)phosphine and its derivatives,diphenyl(methoxy)phosphine and diphenyl(ethoxy)phosphine.

Catalysts and ligands for the hydroformylation process are described byway of example in WO 2010/030339 A1, WO 2008/071508 A1, EP 982 314 B1,WO 2008/012128 A1, WO 2008/006633 A1, WO 2007/036424 A1, WO 2007/028660A1, WO 2005/090276 A1, and for disclosure purposes reference is made tothe said publications, and the catalysts and ligands disclosed thereinare incorporated into this application. The said publications alsodescribe reaction conditions, which are likewise incorporated into thepresent application.

The hydroformylation of ethene uses carbon monoxide and hydrogen,usually in the form of a mixture known as synthesis gas. The compositionof the synthesis gas used for the hydroformylation process can varywidely. The molar ratio of carbon monoxide and hydrogen is generallyfrom 2:1 to 1:2, in particular about 45:55 to 50:50.

The temperature in the hydroformylation reaction is generally in therange of about 50 to 200° C., preferably about 60 to 190° C., inparticular about 90 to 190° C. The reaction is preferably carried out ata pressure in the range of about 5 to 700 bar, preferably 10 to 200 bar,in particular 15 to 60 bar. The reaction pressure can be varied,depending on the activity of the hydroformylation catalyst used.

Suitable pressure-resistant reaction apparatuses for thehydroformylation process are known to the person skilled in the art.Among these are the well-known reactors for gas-liquid reactions, e.g.gas-circulation reactors, bubble columns, etc., which can optionallyhave dividing internals.

Other preferred embodiments of a hydroformylation reaction are describedinter alia in EP 1 294 668 B1, and the contents of the said publicationare hereby incorporated by way of reference into the presentapplication.

According to one particularly preferred embodiment, methacrolein can beproduced from propanal and formaldehyde in a tandem reaction wherepropanal is obtained by the reaction of ethylene, carbon monoxide andhydrogen and is directly reacted with formaldehyde. This process isdescribed in detail by Deshpande et al., Biphasic catalysis for aselective oxo-Mannich tandem synthesis of methacrolein, Journal ofMolecular Catalysis A: Chemical 211 (2004) 49-53,doi:10.1016/j.molcata.2003.10.010, and in U.S. Pat. No. 7,141,702 B2,and said publications are hereby incorporated by way of reference intothe present application.

Step B)

According to the invention, the methacrolein obtained in step A) isreacted in a direct oxidative esterification reaction to give MMA.

For the purposes of the present invention, a direct oxidativeesterification reaction is a process in which methacrolein is reacteddirectly, i.e. without formation of large amounts of methacrylic acid,to give MMA in the presence of methanol and of an oxidant, preferablyoxygen.

In contrast to this, in processes which are carried out by way ofexample by BASF, methacrolein is first oxidized to give methacrylicacid, which is then esterified in a further reaction step with methanolto give MMA.

In the process according to BASF it is possible that methanol presentfor stabilization purposes or present by virtue of the productionprocess in the formaldehyde used likewise forms small amounts of MMAduring the oxidation of methacrolein. However, this MMA cannot beseparated from returned methacrolein by inexpensive purificationprocesses, and generally decomposes under the selected conditions; thistherefore reduces the final yield of MMA from methanol present in theformaldehyde. To solve this problem, a complicated method must be usedto free formaldehyde from methanol. Alternatively, it is certainlypossible to use a purification step to remove the MMA present in thecomposition intended for return, but both solutions incur high cost,which is uneconomic because the amounts of MMA attained are relativelysmall.

Oxidation of methacrolein in an oxidative esterification reactionaccording to step B) of the process of the present invention generallyproduces at most 30% by weight, preferably at most 15% by weight,particularly preferably at most 5% by weight, of methacrylic acid.

An oxidative esterification reaction is carried out with an oxidant, andit is preferable to use oxygen (O₂) for this purpose. For reasons ofcost, air can preferably be used, and can comprise different proportionsof oxygen; this would not be critical for the present invention.

At least one heterogeneous oxidation catalyst is moreover used forcarrying out a reaction according to step B), and these selectivelyaccelerate the oxidation reaction defined in more detail above. Suitablecatalysts are well known to persons skilled in the art and are describedby way of example in the publications EP 0 857 512 A1, EP 1 393 800 A1,EP 2 177 267 A1, and EP 2 210 664 A1, and reference is made to the saidpublications for disclosure purposes, and the catalysts disclosedtherein are incorporated into this application. Said publications alsodescribe reaction conditions, which are likewise incorporated into thepresent application.

Heterogeneous oxidation catalysts preferably comprise at least one noblemetal and/or at least one metal oxide. Preference is given here tooxidation catalysts in which gold and/or palladium and/or rutheniumand/or rhodium and/or silver is present. Gold- and/orpalladium-containing catalysts are particularly preferred.

Among the suitable catalysts for carrying out the present process areinter alia palladium catalysts, where these preferably comprisepalladium and lead and are generally used on a support.

A palladium catalyst can also comprise at least one compound selectedfrom the group consisting of an alkali metal compound and of an alkalineearth metal compound. It is preferable that a palladium catalystcomprises from 0.01 to 30% by weight, more advantageously from 0.01 to5% by weight, of at least one compound selected from the group whichconsists of an alkali metal compound and of an alkaline earth metalcompound.

The introduction of the alkali metal compound and/or of the alkalineearth metal compound into the catalyst can be carried out by a processin which a compound of this type is added to a solution in which apalladium compound and/or a lead compound is present, and a support istreated with the solution, whereupon the alkali metal compound and/orthe alkaline earth metal compound, together with the palladium compoundand/or with the lead compound, is adsorbed onto the support or adheresthereto. Alternatively, a support which comprises an alkali metalcompound and/or an alkaline earth metal compound adsorbed thereon can beused for producing a catalyst. Another possibility, instead of the useof a support, is that a solution in which an alkali metal compoundand/or an alkaline earth metal compound is present is added to thereaction mixture during the reaction according to step B).

The amount of the palladium supported on the support is not subject toany particular restriction, but the amount is preferably from 0.1 to 20%by weight, more preferably from 1 to 10% by weight, based on the weightof the support. The amount of the lead supported on the support is notsubject to any particular restriction, but the amount is preferably from0.05 to 17% by weight, more preferably from 0.45 to 8.5% by weight,based on the weight of the support. The atomic ratio of palladium tolead is preferably in the range from 3:0.7 to 3:1.3, more preferably inthe range from 3:0.9 to 3:1.1.

A disadvantage of the palladium catalysts described above is that thelead usually present can cause environmental harm and therefore thatwastewater requires complicated treatment to remove lead residues.

In an alternative embodiment, a catalyst used for the oxidativeesterification comprises minimized amounts of lead. Accordingly, theoxidative esterification reaction according to step B) can be carriedout with use of a catalyst of which the lead content is preferably atmost 20% by weight, with preference at most 10% by weight, specificallypreferably at most 5% by weight, particularly preferably at most 2% byweight and very particularly preferably at most 1% by weight. Accordingto one specifically preferred embodiment, step B) uses a catalyst whichpreferably comprises at most 1.5% by weight, with preference at most 1%by weight, specifically preferably at most 0.5% by weight, particularlypreferably at most 0.2% by weight and very particularly preferably atmost 0.1% by weight, of lead. Step B) can moreover use a catalyst whichcomprises no measurable content of lead.

It can also be provided that the oxidative esterification reactionaccording to step B) is carried out with use of a catalyst whichcomprises one or more metals selected from the group consisting of goldand/or palladium and/or ruthenium and/or rhodium and/or silver. It ispreferable that said metals take the form of ultra-finely dispersedmetals, i.e. take the form of nanoparticles, applied to a support. Theaverage diameter of the metal particles is preferably at most 20 nm,preferably at most 10 nm, particularly preferably 5 nm, where said valueis based on a numeric average determined by TEM (Transmission ElectronMicroscopy). The “average diameter” of the particles is calculated fromthe diameter of 100 particles selected from 120 particles, where the tenlargest particles and the ten smallest particles of the 120 particlesare ignored.

A preferred catalyst can comprise other catalytically activeconstituents alongside gold particles and/or palladium particles and/orruthenium particles and/or rhodium particles and/or silver particles.Among the other catalytically active constituents are inter aliamagnesium, scandium, yttrium, lanthanum and other lanthanoids withatomic numbers from 58 to 71, titanium, zirconium, hafnium, vanadium,niobium, tantalum, chromium, molybdenum, tungsten, manganese, rhenium,iron, osmium, cobalt, iridium, nickel, platinum, copper, zinc, cadmium,aluminium, gallium, indium, thallium, germanium, tin, lead, antimony,and bismuth, these being able to be respectively present in metallicand/or oxidized form (e.g. in the form of oxides, hydroxides, or salts).Said other catalytically active constituents take the form of particleswhich preferably have an average diameter of at most 20 nm, withpreference at most 10 nm, with particular preference at most 5 nm. It ispossible here that the gold particles and/or palladium particles and/orruthenium particles and/or rhodium particles and/or silver particles andthe particles with other catalytically active constituents are presenttogether or separately, in particular in alloyed or non-alloyed form, onthe support. It is preferable that the gold particles and/or palladiumparticles and/or ruthenium particles and/or rhodium particles and/orsilver particles comprise the other catalytically active constituents.

The proportion of the catalytically active particles of the catalyst canvary widely. The proportions of the catalytically active particles arepreferably in the range of about 0.01 to 20 parts by weight,particularly preferably in the range from 0.1 to 10 parts by weight, per100 parts by weight of the catalyst.

If the catalyst comprises, alongside gold particles, other elements inthe form of catalytically active components, the atomic ratio of gold tothe entirety of the other elements can be in the range from 1:0.001 to1:1000, preferably from 1:0.01 to 1:100, particularly preferably from1:0.1 to 1:10 and specifically preferably from 1:0.2 to 1:5.

It can also be provided that the oxidative esterification reactionaccording to step B) is carried out with use of a nickel-containingcatalyst. It is preferable that nickel-containing catalysts comprisecontent of a noble metal, preferably gold. It is preferable thatnickel-containing catalysts comprise nickel oxide, which is used incombination with nickel, palladium, platinum, ruthenium, gold, silverand/or copper. It is preferable that the atomic ratio of NiO_(x) to(NiO_(x)+X) is in the range from 0.20 to 0.99, preferably from 0.30 to0.90 and particularly preferably from 0.50 to 0.90, where X is selectedfrom nickel, palladium, platinum, ruthenium, gold, silver and/or copper,particular preference being given here to gold. In this formula, NiO_(x)is present as oxide, while X is present in metallic form.

Nickel oxide (NiO_(x)) can by way of example take the form of Ni₂O, NiO,NiO₂, Ni₃O₄ or Ni₂O₃.

The nickel oxide and other components, in particular metalliccomponents, such as nickel, palladium, platinum, ruthenium, gold, silverand/or copper, here can preferably be used in the form of nanoparticleswith a size in the range from 2 to 15 nm, preferably from 2 to 10 nm andparticularly preferably from 2 to 6 nm, where this value refers to anumeric average determined by TEM (Transmission Electron Microscopy), asdefined in more detail above. The nanoparticles are preferably fixed ona support.

The catalysts described above are generally applied on a support, wherethe said supports can comprise metal oxides (e.g. silicon dioxide,aluminium oxide, titanium oxide, zirconium oxide, or magnesium oxide),mixed oxides (such as silicon dioxide-aluminium oxide, titaniumdioxide-silicon dioxide or silicon dioxide-magnesium oxide), zeolites(such as ZSM-5), mesoporous silicates (such as MCM-41), natural minerals(such as clay, diatomaceous earth or pumice), or carbon materials (e.g.activated charcoal or graphite). It is preferable to use inorganicsupports based on oxides.

It is particularly advantageous to use an inorganic support based onoxides and comprising silicon, lithium, sodium, potassium, magnesium,calcium, scandium, yttrium, lanthanum and other lanthanoids with atomicnumbers from 58 to 71, titanium, zirconium, hafnium, vanadium, niobium,tantalum, chromium, molybdenum, tungsten, manganese, rhenium, iron,ruthenium, osmium, cobalt, rhodium, iridium, nickel, platinum,palladium, copper, silver, zinc, cadmium, boron, aluminium, gallium,indium, thallium, germanium, tin, lead, antimony, bismuth and/ortellurium.

A preferred support based on oxides comprises silicon oxide as maincomponent and one or more members of the group consisting of lithium,sodium, potassium, magnesium, calcium, scandium, yttrium, lanthanum andother lanthanoids with atomic numbers from 58 to 71, titanium,zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum,tungsten, manganese, iron, cobalt, nickel, copper, silver, zinc,cadmium, aluminium, gallium, indium, thallium, germanium, tin, lead,antimony and bismuth.

The process for producing the abovementioned inorganic supports based onoxides is not subject to any particular restrictions, and it is possibleto use any known production process. Examples comprise impregnation,coprecipitation, ion exchange, gas-phase deposition, kneading andhydrothermal synthesis.

It is preferable to use a porous support. It is particularly preferablethat the specific surface area (BET method) is generally at least 50m²/g, with preference at least 100 m²/g.

The process for loading the support with the catalytically activeconstituents is not subject to any particular restrictions. Suitableprocesses are inter alia coprecipitation, precipitative deposition,impregnation and vapour-phase deposition.

The catalysts described above based on gold and/or nickel oxide arepreferred over the palladium catalysts. Nickel-containing andgold-containing catalysts can preferably be lead-free.

It can preferably be provided that the water content of the reactionmixture used for the oxidative esterification in step B) is preferablyat most 10% by weight and with preference at most 5% by weight.

The said low proportions of water can optionally be achieved by using aphase separator, and the water content of the methacrolein phase canvary here as a function of temperature. Accordingly, it is preferablethat the reaction mixture obtained after the reaction of formaldehydewith propanal is cooled to a temperature at which the water content inthe methacrolein phase adopts the values mentioned. The temperature inthe phase separator can preferably be adjusted to from 0 to 50° C.,preferably from 5 to 30° C. and particularly preferably from 10 to 25°C. However, removal of the water is only necessary at particularly highwater contents of more than 10% by weight, to the extent that a markedincrease in space-time yield can be achieved. In the case of watercontents above 5% by weight, a small methyl methacrylate yield increasecan be achieved by removing water until content is below 5% by weight.

Equally surprisingly, it has been found that, contrary to theassumptions made in the prior art, high methyl methacrylate space-timeyields can also be achieved with residual content of starting materialsor by-products from step A): although propanal, formaldehyde and dimericmethacrolein are reacted to give methyl propionate, methyl formate andthe methyl ester of oxidized dimeric methacrolein, this formation ofby-products is the only adverse effect of these components on the entireprocess. Surprisingly, therefore, the total yield of MMA is very high,and the by-products mentioned can easily be removed during MMA work-up.

It is also preferable that the content of methacrolein in the reactionmixture used for the oxidative esterification in step B) is at least 5%by weight, preferably at least 15% by weight and particularly preferablyat least 25% by weight.

It can also be provided that the oxidative esterification reactionaccording to step B) preferably takes place with a molar ratio ofmethanol to methacrolein in the range from 1:1 to 50:1, particularlypreferably from 1.5:1 to 25:1 and specifically preferably from 2:1 to10:1.

The amount of the catalyst to be used varies, depending on thecomposition of the feed mixture and of the catalyst, on the reactionconditions, and on the types of reaction and the like. If the catalystused takes the form of a slurry, it is preferable that the amount usedof the catalyst is from 0.01 to 0.5 kg/I of the reaction systemsolution.

The oxidative esterification reaction can be carried out in anyconventional manner, for example in a liquid-phase reaction ortrickle-bed reaction. By way of example, it is possible to use any knownreactor, e.g. a bubble-column reactor, a tubular reactor with air streamor a stirred reactor.

The pressure at which the said reaction is carried out can vary widely.Surprising advantages can be achieved through a reaction pressure in therange from 2 to 100 bar, preferably from 3 to 80 bar, more preferablyfrom 4 to 50 bar and particularly preferably from 5 to 20 bar.

It is preferable to keep the reaction system at a pH of from 5 to 9,particularly from 6.5 to 8, by adding at least one basic compoundpreferably selected from the group consisting of an alkali metalcompound and/or an alkaline earth metal compound, for example an oxide,hydroxide, carbonate, carboxylate or the like.

The oxidative esterification reaction according to step B) can takeplace at a temperature in the range of preferably 10° C. to 200° C.,particularly preferably 40 to 150° C. and specifically preferably 60 to120° C.

The reaction time or residence time varies, depending on other reactionconditions; however, it is preferably in the range from 10 minutes to 48hours, with preference from 30 minutes to 24 hours and particularlypreferably from 45 minutes to 2 hours.

Further information concerning the conduct of an oxidativeesterification reaction according to step B) for MMA synthesis is foundinter alia in U.S. Pat. No. 4,249,019 or DE 3018071A1.

Oxidative esterification under the conditions mentioned above gives areaction mixture which comprises MMA as main reaction product. Theresultant reaction mixture also comprises, in addition to MMA, unreactedmethacrolein and unreacted methanol and small amounts of water andmethacrylic acid as by-products. The reaction mixture also comprisestraces of other by-products, which comprise dimethacrolein and the like.

The reaction product obtained in step B) can be worked up in a knownmanner in order to obtain pure MMA: the reacted reaction mixtureobtained through the oxidative esterification according to step B) canfirst be worked up by distillation.

According to one preferred embodiment, the reaction mixture can bepassed into a distillation tower, and is preferably passed into thecentral portion thereof: it is generally possible to discharge anazeotropic mixture of methacrolein and methanol as overhead distillationproduct.

A mixture comprising liquid MMA, methanol, water and other by-productsis obtained from the bottom of the distillation tower. Said liquidmixture is purified by a conventional process. Said purification unitcan generally comprise at least one, preferably two or more,distillation systems for removing high-boiling-point andlow-boiling-point compounds.

In the present invention, the nature of the distillation tower that ispreferably used for work-up of the reaction mixture obtained from theoxidative esterification is not subject to any particular restriction,and it is possible to use any desired conventional distillation tower,for example a plate column or a packed column.

However, since methacrolein, MMA and methacrylic acid, which are passedinto the distillation tower, are readily polymerizable compounds it ispreferable to use a distillation system with a structure where blockageby polymerization products does not occur and/or the polymerizationproducts can easily be removed. Specific examples of distillation towerscomprise plate columns equipped with a sieve tray, cascade tray,turbogrid tray, grooved tray or the like, and packed columns withordered packing materials (e.g. Mellapak from Sulzer) or with unorderedpacking materials (e.g. Raschig Superring from Raschig).

The suitable distillation temperature in the distillation towerpreferably used in the claimed process for work-up of the reactionmixture obtained from the oxidative esterification varies as a functionof the distillation pressure, of the composition of the liquid in thedistillation tower, of the number of plates in the distillation towerand the like. However, in order to minimize the formation of theabovementioned polymerization products and the formation ofhigh-boiling-point compounds which represent a yield loss, based onmethacrolein or MMA, it is preferable that the distillation temperatureis minimized. However, if a very low distillation temperature isselected disadvantages can arise. Among these are, by way of example,that a low distillation pressure also has to be selected. This canrequire use of a disadvantageously large distillation tower. It canmoreover be necessary to use a coolant for concentrating the gas phasein the uppermost portion of the distillation tower. It is preferablethat the distillation temperature, or the temperature of the liquid inthe column, is in the range from 20 to 100° C., particularly from 40 to85° C. The distillation pressure is calculated from said temperature.

As mentioned previously, methacrolein, MMA and optionally otherpolymerizable by-products, such as methacrylic acid, can be passed intoa distillation tower in order to separate the reaction product of theoxidative esterification into methanol/methacrolein mixtures andMMA/water mixtures.

Because polymers can be formed, it is preferable that one or morepolymerization inhibitors is/are added to the process. Polymerizationinhibitors are well known to persons skilled in the art, examples beinghydroquinones, hydroquinone ethers, such as hydroquinone monomethylether or di-tert-butylpyrocatechol, phenothiazine,N,N′-diphenyl-p-phenylenediamine,4-hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl, p-phenylenediamine,methylene blue and sterically hindered phenols. The said compounds canbe used individually or in the form of mixtures and are generallyavailable commercially. The action of the stabilizers mostly consists intheir action as free-radical scavengers for the free radicals arisingduring polymerization. Reference is made to the familiar technicalliterature for further details, in particular to Römpp-Lexikon Chemie[Römpp's Chemical Encyclopaedia]; Editors: J. Falbe, M. Regitz;Stuttgart, New York; 10th Edition (1996); keyword “Antioxidantien”[Antioxidants] and to the references there cited.

In particular, phenols are preferably used as polymerization inhibitor.Particularly surprising advantages can be achieved when hydroquinonemonomethyl ether is used. The proportion of the inhibitors, individuallyor in the form of a mixture, can generally be from 0.001 to 0.5% byweight, based on the weight of the entire composition.

It is preferable that steps A) and B) are carried out in a continuousprocess. Introduction of starting materials into the plant for carryingout a process according to the present invention, and removal ofproducts from the plant, take place here continuously over any desiredperiod. Said period can be interrupted for maintenance and repair work,however.

It can also be provided that the reactor volume in step A) is smallerthan the reactor volume in step B). The reactor volume here is based onthe volumes in step A) and step B), where the starting materials usedwithin these are reacted in the liquid phase under the elevated pressureof the respective reaction to give the products.

The ratio of reactor volume in step A) to reactor volume in step B) isadvantageously in the range from 1:1000 to 1:100, preferably in therange from 1:800 to 1:200 and particularly preferably in the range from1:500 to 1:300.

Typical reactor volumes for a continuously operated production plant canby way of example be for step A) a tubular/tube-bundle reactor ofcapacity from 0.1 to 0.5 m³ and for step B) a tubular/tube-bundlereactor of capacity from 10 to 15 m³ or a continuously operated stirredtank of capacity from 50 to 100 m³, but these data are not intended torepresent any restriction.

Surprisingly, when the present process is compared with conventionalprocesses which oxidize C4 building blocks such as isobutylene, itsuccessfully achieves a marked reduction in the volume to be compressed,in particular of gas.

In the claimed process, composed of the combination of the reactionsteps A) and B), there is no essential requirement in any of theindividual reaction steps to introduce into the reaction mixtureadditional water, i.e. water that is not already present by virtue ofother circumstances in the reactants. This is a decisive advantage overthe prior art, since additionally introduced water enlarges the reactionstreams and therefore also enlarges the necessary apparatuses, andgenerally also in turn requires removal from the desired final products,with resultant additional energy cost and other costs. Overall reactionvolumes and volume flow rates can therefore be kept low in the claimedprocess.

The Asahi process according to U.S. Pat. Nos. 5,969,178 and 7,012,039requires by way of example during the gas-phase oxidation of theC₄-component the addition and in turn subsequent removal of astoichiometric excess of water. The processes known hitherto forproducing methacrolein from ethylene by way of propionaldehyde, thenoxidizing this to methacrylic acid and then esterifying to MMA requirethe addition of a stoichiometric excess of water during the gas-phaseoxidation of the methacrolein to methacrylic acid, in order to ensurethat the activity of the oxidation catalyst is maintained.

In the claimed process, composed of the combination of the reactionsteps A) and B), the total amount of water separately added duringconduct of the reaction is therefore not greater than 100 mol %,preferably 50 mol %, particularly preferably 30 mol %, and even morepreferably not greater than 10 mol %, based in each case onmethacrolein. In one particularly advantageous embodiment of the claimedprocess, no water is separately added to the reaction mixture in eitherof the reaction steps A) and B) during the conduct of the reaction.Water of reaction and water addition for work-up steps are alwaysexcluded here. FIG. 1 is by way of example a diagram of the claimedprocess, but is not intended to restrict the invention.

Formaldehyde (FA) and propanal (PA) are introduced after premixing orindividually into the reactor 1, and similarly the organic base (OB) andthe acid catalyst (A) are introduced after premixing or individuallyinto the reactor. After aldol condensation and catalyst removal, themethacrolein (MAL) is isolated. The catalyst can be returned to thereactor 1 by way of stream (1). The MAL and methanol (MeOH) areintroduced into the oxidative esterification reactor (DOE reactor). Anoxygen-containing gas (O₂) is fed into the said reactor. The unreactedMAL from the esterification reactor is removed in the form ofMAL/methanol azeotrope in an MMA/water-MAL/MeOH separation process, andis returned to the DOE reactor by way of stream (2). An MMA/waterseparation process and further purification of the crude MMA thenfollow.

FIG. 2 shows a possible system for the reaction of formaldehyde withpropanal to give methacrolein (step A). Aqueous formalin (101) is mixedwith propionaldehyde (102) and passed in the form of stream (103) intothe preheater (11). Dimethylamine (40% aqueous solution) (104) andacetic acid (105) are mixed and passed in the form of stream (106) intothe preheater (12). Operation of the preheaters (11) and (12) isoptional. The mixture from the outlets of (11) and (12) is introduced inthe form of stream (107) into the tubular reactor (13). The tubularreactor (13) is heated by means of an oil bath to the reactiontemperature. Downstream of the tubular reactor, the mixture (108) isdepressurized in valve (14) and introduced into the column (15). Thematerial discharged from the bottom of the column is divided (50/50),and one portion is returned to the stream (107) into the reactor (13),the other portion being passed for disposal in the form of wastewaterstream (112). The stream obtained at the top of the column is condensedin the condenser (16) and is introduced in the form of stream (109) intothe phase separator (17). A methacrolein-rich phase (111) in the saidphase separator is discharged as product into the “direct oxidativeesterification” portion of the system according to FIG. 3, and thestream (111) here can optionally be dried by azeotropic distillation(not shown). The aqueous discharge from the phase separator (17) isreturned in the form of stream (110) to the column (15).

FIG. 3 shows a possible apparatus which can produce MMA from MAL andwhich is suitable for carrying out the direct oxidative esterification(step B). Methanol is introduced by way of supply line (200) to the line(111) by way of which methacrolein is taken from step A) of the process.Air (or oxygen-containing gas mixture) is passed by way of the supplyline (202) into the reactor (21), which comprises a catalyst suitablefor the direct oxidative esterification process, and a basic compositionwhich preferably comprises methanol and NaOH is also passed by way ofthe supply line (203) into the reactor (21) in order to adjust pH. FIG.3 does not show the ancillary assemblies, such as pumps, heatingelements, heat exchangers and condensers. It is also optionally possibleto use a plurality of reactors (21) connected in series (not shown).

Exhaust gases are discharged from the reactor (21) through theexhaust-gas scrubber by way of line (204), and methanol, MMA andmethacrolein here can be at least to some extent condensed in one ormore condensers and returned to the reactor (21) (not shown).

The reaction mixture present in the reactor (21) is passed by way ofline (205) to the distillation column (22), and methacrolein (ormethacrolein-containing mixture) here is returned by way of line (206)to the reactor (21). Gases and other low-boiling-point components can beremoved from the reaction mixture at the top of the distillation column(22) and introduced by way of line (207) into the exhaust gas. Thecomposition taken by way of line (209) from the material at the bottomof the distillation column (22) in essence comprises MMA, which cancomprise methanol, methacrylic acid, sodium methacrylate and othercomponents.

An acid or acid-containing mixture, for example aqueous sulphuric acid,can be passed by way of supply line (208) from a feed vessel to the line(209). The resultant mixture is passed to a water/oil separation system(23) which by way of example can comprise a centrifuge, and is separatedinto aqueous and organic phase. At this point it is possible to connecta plurality of similar water/oil separation systems in parallel, so thatthese can be operated in alternation as required (not shown). Theaqueous phase from the separation system (23) can be introduced by wayof line (210) into a wastewater treatment system, while the organicphase is introduced by way of line (211) to the distillation column (24)for removal of the high-boiling-point components. High-boiling-pointcomponents, e.g. methacrylic acid, can be taken from the material at thebottom of the said column (24) by way of line (212) for furtherpost-treatment. Crude MMA is taken from the top of the column (24) byway of line (213) and introduced into the column (25). Thelow-boiling-point components (e.g. methanol and methacrolein) can betaken from the top of the said column by way of line (215) and returnedby way of line (218) to the reactor (21), and a portion thereof can bedischarged and introduced into the exhaust-gas stream (204). PurifiedMMA can be taken from the material at the bottom of the column (25) byway of line (214) and can be introduced into the final column for MMApurification (26). Pure MMA is taken from the top of the column by wayof line (217), while the remaining high-boiling-point components can betaken from the material at the bottom of the column by way of line (216)for further post-treatment or for return upstream of column (24) (notshown).

Key FIG. 1 OB Organic base A Acid MeOH Methanol O₂ Oxygen-containing gasMMA Methyl methacrylate FA Formalin (aqueous formaldehyde solution) PAPropanal MAL Methacrolein DOE Direct oxidative esterification {circlearound (1)} Catalyst return {circle around (2)} Methacrolein/methanolreturn FIG. 2 FOL Formalin (aqueous formaldehyde solution) PA PropanalDMA Aqueous dimethylamine solution AcOH Acetic acid MAL Methacrolein 11Heat exchanger (preheater) 12 Heat exchanger (preheater) 13 Reactor(tubular reactor) 14 Pressure-retention valve 15 MAL distillation column16 Condenser 17 Phase separator 101 Aqueous formalin line 102 Propanalline 103 Line leading to heat exchanger 104 Dimethylamine (40% aqueoussolution) line 105 Acetic acid line 106 Line leading to heat exchanger107 Line leading to reactor 108 Line taking product mixture to column109 Condensate line 110 Return to column 111 MAL to stage B) 112 Lineleading to wastewater system 113 Return of material discharged frombottom of column FIG. 3 MAL Methacrolein MeOH Methanol O₂Oxygen-containing gas MMA Methyl methacrylate 21 Reactor 22 MAL removal23 Water/oil separation 24 Removal of high-boiling-point materials 25Removal of low-boiling-point materials 26 Final purification of MMA 111MAL from stage A) 200 Methanol supply line 201 MAL/methanol supply line202 Supply line for oxygen-containing gas 203 Supply line for basiccomposition 204 Exhaust-gas stream 205 Product stream to column 22 206MAL return 207 Exhaust-gas stream 208 Acid supply line 209 Productstream to water/oil separation system 210 Aqueous phase discharge line211 Product stream to column 24 212 Discharge line forhigh-boiling-point materials 213 Product stream to column 25 214 Productstream to column 26 215 Discharge line for low-boiling-point materials216 Discharge from bottom of column 217 Pure MMA line 218 Return oflow-boiling-point materials

The examples below serve for further explanation of preferredembodiments of the present invention, but are not intended to restrictthe invention.

EXAMPLE 1

In a system corresponding to FIG. 2, propanal (PA) is reactedcontinuously with formaldehyde with use of dimethylamine (DMA) andacetic acid (AcOH). 251 g/h of PA and 349 g/h of a 37 percent formalinsolution are premixed homogeneously (molar ratio 1:1). 18.7 g/h of acatalyst solution with 24.8% of dimethylamine and 37.9% of acetic acidare passed into the preheater 12. The two streams are heated to atemperature of 170° C. before they are combined. The preheated streamsare combined in a T mixer which has direct connection to a tubularreactor ( 1/16 inch tube, length 4.2 m). The temperature of the reactoris controlled by an oil bath operated at 180° C., residence time is 10s, and the pressure in the tubular reactor is 70 bar. Downstream of thetubular reactor, the mixture is depressurized in valve (14) and isintroduced into the column (15). 335 g/h of the material discharged atthe bottom of the column are returned to the reactor (13), and 370 g/hof the material discharged at the bottom of the column are passed fordisposal in the form of wastewater. After condensation of the overheadstream in the condenser (16) and phase separation in (17), amethacrolein-rich phase with 96.5% methacrolein content is discharged asproduct (111), and the aqueous material discharged from the phaseseparator is returned to the column (15). Conversion is 99.9% and yieldis 98.1%, based on propionaldehyde. The residual water content of themethacrolein used in Examples 2 to 4 was 1.7% by weight.

EXAMPLE 2

Catalyst 1 (0.9% Au-1.1% NiO on SiO₂—Al₂O₃—MgO) was produced by analogywith Example 1 of EP 2 210 664 A1. A solution of 375 g of aluminiumnitrate nonahydrate, 256 g of magnesium nitrate hexahydrate and 54 g of60% nitric acid in 500 mL of water was added dropwise at 15° C. to 2 kgof silica sol solution with particle size from 10 to 20 nm (NissanChemical Industries, Snowdex N-30, 30 wt % SiO₂). The mixture wasstirred at 50° C. for 24 hours, then cooled to room temperature,spray-dried (130° C.) and calcined (300 to 600° C., 10 hours in total).30 g of this SiO₂—Al₂O₃—MgO support were suspended in 100 mL of waterand heated to 90° C. After 15 min at 90° C., this suspension was addedto a solution of 1.64 g of nickel nitrate hexahydrate and 530 mg ofauric acid (HAuCl₄) in 100 mL of water. After stirring at 90° C. for afurther 30 minutes, the mixture was cooled and the solid was removed,then washed three times with 100 mL of fresh water and in each casestirred at 20° C. for 5 minutes and filtered off. The catalyst was driedat 105° C. within a period of 10 hours and calcined at 450° C. within aperiod of 5 hours in air. ICP analysis showed that the resultant violetpowder comprised 1.1% of Ni and 0.9% of Au. The average size of goldnanoparticles (TEM) was less than 5 nm.

A mixture of 0.67 g of methacrolein (from Example 1), 5.65 g of methanoland 504 mg of Au catalyst 1 was stirred in an autoclave under 11 bar ofan O₂/N₂ gas mixture (7% by volume of O₂) at 80° C. within a period of 2hours, and then cooled and filtered and analysed by means of GC. MALconversion was 98.4%, MMA yield was 94.8%, MMA selectivity was 96.3%,and space-time yield was 9.3 mol MMA/kg cat-h.

EXAMPLE 3

Catalyst 2 (1% Au-5% ZnO-5% MgO on SiO₂) was produced by analogy withExample 1-6 of EP1393800A1. 89 g of a commercially available SiO₂support (Cariact Q-10, 75-150 μm, Fuji Silisia) was impregnated with asolution of 18.3 g of zinc nitrate hexahydrate and 12.8 g of magnesiumnitrate hexahydrate in 90 mL of water, dried at 120° C. within a periodof 12 hours, and then calcined at 600° C. within a period of 4 hours.300 mL of a 20 mmol/L solution of HAuCl₄ were adjusted with a 0.5 Msolution of NaOH at 70° C. to pH=7, and the previously producedSiO₂—ZnO—MgO support was added at the said temperature, with stirring.After further stirring at 70° C. for one hour, the mixture was cooledand filtered, and the catalyst was washed three times with 400 mL offresh water, being stirred in each case at 20° C. for 5 minutes. Afterdrying at 100° C. for 10 hours, the material was calcined at 400° C.within a period of 3 hours in air. ICP analysis showed that theresultant violet powder comprised 1.5% Au. The average size of goldnanoparticles was less than 5 nm.

A mixture of 0.60 g of methacrolein (from Example 1), 5.76 g of methanoland 300 mg of catalyst 2 was stirred in an autoclave under 11 bar of anO₂/N₂ gas mixture (7% by volume of O₂) at 80° C. within a period of 2hours, and then cooled and filtered and analysed by means of GC. MALconversion was 85.5%, MMA yield was 83.4%, MMA selectivity was 97.5%,and space-time yield was 14.0 mol MMA/kg cat-h.

EXAMPLE 4

Catalyst 3 (1.5% Au-5% La₂O₃-5% MgO on SiO₂) was produced by analogywith Example 1-7 of EP1393800A1. 88.5 g of a commercially available SiO₂support (Cariact Q-10, 75-150 μm, Fuji Silisia) was impregnated with asolution of 13.3 g of lanthanum nitrate hexahydrate and 12.8 g ofmagnesium nitrate hexahydrate in 90 mL of water, dried at 120° C. withina period of 12 hours, and then calcined at 600° C. within a period of 4hours. 450 mL of a 20 mmol/L solution of HAuCl₄ were adjusted with a 0.5M solution of NaOH at 70° C. to pH=7, and the previously producedSiO₂—La₂O₃ support was added at the said temperature, with stirring.After further stirring at 70° C. for one hour, the mixture was cooledand filtered, and the catalyst was washed three times with 400 mL offresh water, being stirred in each case at 20° C. for 5 minutes. Afterdrying at 100° C. for 10 hours, the material was calcined at 400° C.within a period of 3 hours in air. ICP analysis showed that theresultant violet powder comprised 1.5% Au. The average size of goldnanoparticles was less than 5 nm.

A 42.9% solution of methacrolein (from Example 1) in methanol wasintroduced continuously at a flow rate of 420 g/h into a mechanicallystirred 2.5 L stirred-tank reactor with a catalyst separator to which255 g of catalyst 3 had been charged. An NaOH solution in methanol (from1 to 4% by weight) was added at a flow rate of 40 g/h to maintain the pHof the reaction mixture at about 7. The amount of air introducedcontinuously into the reactor at 5 bar and 80° C. was sufficient to givea residual proportion of oxygen in the exhaust gas of about 4% by volumeof O₂. The product mixture taken continuously from the system wasanalysed by means of GC. 50 h after the start of the process,methacrolein conversion was 78.5%, MMA yield was 76.5%, MMA selectivitywas 97.4%, and space-time yield was 9.8 mol MMA/kg cat-h. The reactionmixture obtained in the reactor (21) was introduced at level 30 (fromabove) into the distillation column (22) (45 levels, diameter 15 cm,height 6 m). The temperature of the material at the bottom of the columnwas 84° C., and the overhead temperature was 31° C. Themethacrolein/methanol mixture was taken from the column at level 5 (fromabove) and returned to the reactor (21). The composition taken from thematerial at the bottom of the distillation column (22) at a flow rate of500 g/h was composed of MMA which also comprised methanol, methacrylicacid, sodium methacrylate and other components. The amount of a 10%aqueous sulphuric acid continuously introduced from a feed vessel intothe said stream (line 209) was sufficient to give pH=2 in the resultantmixture. The resultant mixture was separated by means of a centrifuge inthe water/oil separation system (23). The organic phase was introducedat level 20 (from above) at a flow rate of 375 g/h into the distillationcolumn (24) (30 levels, diameter 10 cm, height 5 m). The said column wasoperated at 150 mmHg, the temperature of the material at the bottom ofthe column being 70° C. and the overhead temperature being 45° C. CrudeMMA was taken from the top of the column (24) at a flow rate of 288 g/hand was introduced at level 10 (from above) into the column (25) (30levels, diameter 10 cm, height 5 m). The column (25) was operated at 250mmHg, the temperature of the material at the bottom of the column being80° C. and the overhead temperature being 50° C. Purified MMA was takenat a flow rate of 281 g/h from the material at the bottom of the column(25) and was introduced at level 35 (from above) into the column (26)(70 levels, diameter 10 cm, height 5 m). The column (26) was operated at140 mmHg, the temperature of the material at the bottom of the columnbeing 80° C. and the overhead temperature being 55° C. Pure MMA wastaken from the top of the column (26) at a flow rate of 250 g/h. Thetotal yield of methyl methacrylate isolated from methacrolein was 97.2%after the work-up described above.

TABLE 1 Summary of reactions for steps A) and B): Space-time Overallyield, step B selectivity [mol MMA/kg of PA Example Reaction YieldSelectivity cat-h] to MMA 1 PA-MAL 98.1 98.2 2 MAL-MMA 94.8 96.3 9.394.6 3 MAL-MMA 83.4 97.5 14.0 96.0 4 MAL-MMA 76.5 97.4 9.8 95.6

Comparative Values From the Prior Art:

The following Comparative Examples (Comparison 5-8) collate possibleprocesses and combinations of various processes according to the priorart with the corresponding selectivities of the individual steps and theoverall processes.

COMPARATIVE EXAMPLE 5

Reaction A: Isobutylene→Methacrolein

Reaction B: Methacrolein→Methacrylic acid

Reaction C: Methacrylic acid→Methyl methacrylate

C=conversion

S=selectivity

Reaction A (e.g. U.S. Pat. No. 7,012,039B2, Example 1, pressure <2 bar):

S(MAL)=88.3%,

S(MAA)=2.4%

Water addition H₂O/MAL=1.7 (mol/mol)

Reaction B, (e.g. EP0376117B1, Example 1, pressure <2 bar)

S(MAA)=89.2%

Water addition H₂O/MAL=5.5 (mol/mol)

Reaction C (e.g. US20020188151A1)

S(MMA)˜100%

Overall selectivity of reactions A+B+C=S(MAL+MAA from IBN)*S(MAA fromMAL)*S(MMA from MAL)=80.9%

COMPARATIVE EXAMPLE 6

Reaction A: Isobutylene→Methacrolein

Reaction B: Methacrolein→Methyl methacrylate

Reaction A (e.g. U.S. Pat. No. 7,012,039B2, Example 1, pressure <2 bar):

S(MAL)=88.3%,

S(MAA)=2.4%

Water addition H₂O/MAL=1.7 (mol/mol)

Reaction B, variant 1 (U.S. Pat. No. 7,012,039B2, Example 1):

S(MMA)=90.7%

Reaction B, variant 2 (EP 2 210 664 A1, Example 7):

S (MMA)=97.2%

Overall selectivity A+B (variant 1): S(MMA from IBN)=S(MAL)*S(MMA)=80.1%

Overall selectivity A+B (variant 2): S(MMA from IBN)=S(MAL)*S(MMA)=85.8%

COMPARATIVE EXAMPLE 7

Reaction A: Propionaldehyde→Methacrolein

Reaction B: Methacrolein→Methacrylic acid

Reaction C: Methacrylic acid→Methyl methacrylate

Reaction A (e.g. DE3213681A1, Example 1)

S(MAL)=98.1%,

Reaction B (e.g. EP0376117B1, Example 1, pressure <2 bar)

S(MAA)=89.2%

Water addition MAL/H₂O=5.5 (mol/mol)

Reaction C (e.g. US20020188151A1) S(MMA)˜100%

Overall selectivity of steps A+B+C=S(MMA from PA)=S(MAL from PA)*S(MAAfrom MAL)*S(MMA from MAA)=87.5%

COMPARATIVE EXAMPLE 8

Reaction A: Propionaldehyde→Methacrolein

Reaction B: Methacrolein→Methyl methacrylate

Reaction A, (CN101074192A1, pressure <2 bar)

S(MAL)=95.2%

Reaction B, (CN101074192A1)

S(MMA)=98.2%

Overall selectivity of steps A+B: S(MMA from PA)=S(MAL)*S(MMA)=93.5%

TABLE 2 Comparison of processes (part 1) Comparative Overall Extra H₂Oexamples Reaction selectivity [mol/mol MMA] 5 IBN-MAL-MAA-MMA 80.9 7.2 6IBN-MAL-MMA 80.1 1.7 85.8 7 PA-MAL-MAA-MMA 87.5 5.5 8 PA-MAL-MMA 93.5 —4 PA-MAL-MMA 95.6 —

Comparative processes 5 to 7 in particular require large molar amountsof separately added water during the reaction steps, while overallselectivities are markedly poorer.

TABLE 3 Comparison of processes (part 2) Cat. for Cat. for MAL MMAExample/ synthesis synthesis Comparative Overall [mol amine/ [mol MMA/kgexample Reaction selectivity mol MAL] cat-h] 8 PA-MAL-MMA 93.5 1.040 3.74 PA-MAL-MMA 95.6 0.024 9.8

As shown in the table above, the reaction according to the invention(Example 4) can be carried out in step A) with catalytic amounts ofamine base, based on MAL, while Comparative example 8 requires astoichiometric excess of catalyst for this purpose. The value of 9.8 forthe space-time yield of step B) from the claimed process is likewisemore than twice as high as in Comparative example 8.

The invention claimed is:
 1. A process for producing methylmethacrylate, the process comprising: A) producing methacrolein frompropanal and formaldehyde and B) reacting the methacrolein in anoxidative esterification reaction to obtain methyl methacrylate, whereinthe producing A) and the reacting B) take place in a liquid phase at apressure of from 2 to 100 bar, and the reacting B) is carried out in thepresence of a heterogeneous noble-metal-comprising catalyst which doesnot comprise palladium and which comprises at least one selected fromthe group consisting of gold and nickel.
 2. The process according toclaim 1, wherein the producing A) is carried out in the presence of from0.1 to 20 mol % of organic base and from 0.1 to 20 mol % of acid, basedin each case on propanal.
 3. The process according to claim 1, whereinthe producing A) is carried out at a temperature of from 100 to 300° C.4. The process according to claim 1, wherein the producing A) is carriedout at a pressure of from 5 to 100 bar.
 5. The process according toclaim 1, wherein the heterogeneous oxidation catalyst comprises one ormore ultra-finely dispersed metals with an average particle size of <20nm selected from the group consisting of gold and nickel.
 6. The processaccording to claim 2, wherein the organic base comprises a secondaryamine.
 7. The process according to claim 2, wherein the acid comprisesat least one organic acid.
 8. The process according to claim 2, whereina molar ratio of the acid to the organic base ranges from 20:1 to 1:20.9. The process according to claim 1, wherein the producing A) is carriedout with a residence time ranging from 0.1 to 300 seconds.
 10. Theprocess according to claim 1, wherein the reacting B) is carried outwith a pressure ranging from 2 to 50 bar.
 11. The process according toclaim 1, wherein the oxidative esterification reaction is carried out ata temperature ranging from 10 to 200° C.
 12. The process according toclaim 1, wherein the oxidative esterification reaction takes place inthe presence of methanol with a molar ratio of methanol to methacroleinranging from 1:1 to 50:1.
 13. The process according to claim 1, whereinthe producing A) and the reacting B) are carried out in a continuousprocess.
 14. The process according to claim 1, wherein a volume ratio ofa reactor in the producing A) to a reactor in the reacting B) rangesfrom 1:1000 to 1:100.
 15. The process according to claim 1, wherein atotal amount of any separately added water in the producing A) and thereacting B) during the process is not greater than 100 mol %, based onmethacrolein.
 16. The process according to claim 1, wherein no water isseparately added in the producing A) and the reacting B) during theprocess.
 17. The process according to claim 1, wherein a content ofwater in reaction mixture during the reacting B) is at most 10% byweight based on a total weight of the reaction mixture.
 18. The processaccording to claim 17, wherein no dewatering is conducted after theproducing A) and before the reacting B).
 19. The process according toclaim 1, wherein the heterogeneous noble-metal-comprising catalystcomprises nickel.